CNC Routing Technique for Milling Interior Corners

T-Bolt connection using laser-cut 1/4″ panels

Laser cutting is a great technique for quick fabrication, in part because of the very small point size of the laser beam. The minuscule kerf of a laser cut allows for very tight 90° angles on interior (and exterior cuts). This makes it easy to laser-cut thin panels of material for joining, as oomout demonstrates in this post on instructables.com (example pictured below). One of the drawbacks of laser cutting is the limited depth of materials that can be successfully through-cut; the laser cutter that I have access to can get through about 1/2″ of MDF or plywood, but the cuts are very charred and not perpendicular due to the hourglass shape of the laser’s focal point.

CNC routing is another great technique for quick fabrication. The range of material depth is much greater, there’s no burning, and through-cuts are perpendicular. One of the biggest drawbacks, however, is the inability to route interior corners due to the cylindrical shape of the cutting tool. This can require tedious hand-finishing to file or chisel out the interior corners to 90° angles, which is not worthwhile for projects that don’t have to look pretty, i.e. prototypes.

90° joint using laser-cut 1/2″ MDF – note the charring

T-bolt connection using laser-cut 1/2″ MDF – note the charring

90° joint using 1/2″ CNC-routed MDF

90° joint using 1/2″ CNC-routed MDF

Purple = cut piece; Blue = interior corner knockouts

Cutout sizing

T-Bolt connection using laser-cut panels

The technique pictured above is achieved by simply routing an extra bit of material out of interior corners to allow for 90° butt-joints. To set the paths for the corner knock-outs, a circle is created at each interior corner using a 3-point definition; the first point is set on the corner’s intersection, the second two are placed on each edge of the corner at a distance equal to the diameter of the router bit you plan on using. This will make sure enough material is taken away to allow for a 90° joint.